The present invention relates to fiber optic sensors. Specifically, the invention relates to optical devices for use in telecommunications and fiber optic sensors.
Optical fibers have been developed having an asymmetric cross-section, as in U.S. Pat. No. 4,669,814 to Dyott. Such fibers may be prepared with one side of the optical fiber located near the optical guiding region. The guiding region of the fiber may have a non-circular cross-section defining two transverse orthogonal axes, which, in combination with the different refraction indices of the core and cladding, may permit the de-coupling of waves polarized along said axes. The non-circular cross-section of the outer surface of the fiber may have a predetermined geometric relationship to the transverse axes of the guiding region, so that the orientation of those axes may also be ascertained from the geometry of the outer surface. Such optical fibers may be geometrically induced birefringent polarization-preserving fibers. Asymmetric fibers with circular cross-section cores may find applications as well, as in U.S. Pat. No. 4,815,817 to Levinson.
Asymmetric fibers may be used in many fiber optic devices, for example, indium-coated polarizers as in U.S. Pat. No. 4,712,866 to Dyott, grating filters as in U.S. Pat. No. 6,075,915 to Koops, et al., and many sensor arrangements. For some of these devices, the asymmetric fiber may be used to permit access to the fields of the optical waveguide. In practice, however, the core-to-surface distance may be large enough to prevent external interaction with the evanescent tails of the optical mode field. In U.S. Pat. Nos. 5,854,864 and 6,047,095, both to Knoesen et al., an asymmetric fiber is polished until only a thin layer of cladding remains covering the core so as to form an evanescent coupling region. In U.S. Pat. No. 6,185,033 to Bosc et al., an electrode is placed adjacent an exposed core of an optical fiber. The devices recited in these patents may provide better access to the fields of the optical waveguide. However, coupling and alignment considerations may remain, as evanescent field interactions are weak and may thus require the interaction length to be long or the amount of control voltage to be high. In devices coupling light out of the core by having an electro-optic material of higher refractive index than the core, the light must be coupled back into the core. This is necessarily wavelength selective, though most devices are desired to be broadband. Further, the refractive index of the electro-optic material is a function of temperature, which function may not normally be the same as that for the core. The performance of such devices may thus be very temperature dependent, requiring such devices to be temperature stabilized. Thus, a method for permitting direct access to the fields of the optical waveguide would find use in a wide variety of fiber optic devices, including those devices mentioned previously as well as other fiber optic applications, such as telecommunications and sensors.
According to one aspect of the invention, fiber material may be removed from an optical fiber to expose the fiber core and the core may then be at least partially removed. One or more optical materials may then be incorporated into the core area to replace the removed core. As used herein, an optical material may include any of numerous materials that may be optically transmissive of light propagating within the fiber, and/or may have optically useful properties. In one embodiment, the fiber material and core may be removed by etching. In another embodiment, the fiber material may be removed by side polishing and the core removed by etching. In a further embodiment, the fiber material and/or core may be removed by excavation with an eximer laser. Other embodiments may include fiber material and/or core removal by Reactive Ion Etching and other methods as are known in the art.
According to another aspect of the invention, an asymmetric fiber may be etched until one side of the fiber may be near the core. In one embodiment, the fiber is further etched on a selected portion of the side near the core to at least partially remove the core and at least one optical material may then be incorporated into the core area to replace the removed core. In another embodiment, the core may be excavated by an eximer laser to at least partially remove the core.
According to another aspect of the invention, a circular fiber may be side polished until the core may be nearly exposed. In one embodiment, the core may then be at least partially removed by etching the side polished face. In another embodiment, the core may be at least partially removed by excavation with an eximer laser. The removed core material may then be replaced by at least one optical material.
According to another aspect of the invention, material may be removed from an optical fiber to expose the fiber core and the core may then be at least partially removed. In one embodiment of the invention, the removed core material may then be replaced with an electro-optic material. In another embodiment of the present invention, the removed core material may be replaced with a rare-earth doped material. In yet another embodiment of the present invention, the removed core material may be replaced with a thermo-optic material. In a further embodiment of the present invention, the removed core material may be replaced with a combination of optical materials.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.